The invention relates to a multi-layer polypropylene film structure having comparatively low film to film coefficient of friction and superior heat seal characteristics.
Highly crystalline polypropylene film is an excellent packaging material, but it has a high film to film coefficient of friction which makes it difficult or impossible for it to be successfully utilized in automatic packaging equipment. The film will not respond to the packaging speed capability of the system and, as a consequence, jamming results. This material in film form also has a tendency to stick at heat seal surfaces, such as, in packages requiring hot crimp seals. Thus, it is impossible to employ this material in standard commercially available horizontal or vertical form and fill packaging systems without first modifying the polypropylene film so that it can be handled at practicle packaging speeds.
In the past, the coefficient of friction characteristics of polypropylene and other thermoplastic films have been benefically modified by the inclusion in the polymer of coefficient of friction (COF) additives such as, for example, fatty acid amides. The effectiveness of an amide depends upon its ability to migrate to the surface of the films in order to reduce COF. While such amides do improve the COF of the films, the value of the COF is subject to wide variation depending upon the heat history which the film experiences during storage, shipping, and certain converting processes. The presence of such amides on the film surfaces can adversely affect the film's appearance as manifested by an increase in haze, a decrease in gloss and the presence of streaks. The presence of such amides on the surface can also adversely affect the wettability and adhesion of solvent and water base inks, coatings and adhesives.
It is an object of the present invention to provide a multilayer polypropylene film having a comparatively low coefficient of friction, as compared with the unmodified film, without the use of prior art COF additives or anti-block agents.
It is yet another object of the present invention to provide a film having improved COF characteristics which are stable with regard to the heat history of the film.
It is a further object of the present invention to provide a film having improved COF characteristics without the adverse affects on appearance, wetting and adhesion typically experienced with additive-modified films.